1. Field of the Invention
The invention relates to a ball joint including a socket connected to a shaft and a ball fitted in the socket, and a steering device including the same.
2. Description of Related Art
Japanese Patent Application Publication No. H11-342854 (JP H11-342854 A) describes an example of a steering device including a ball joint. The steering device includes a rack shaft, and a ball joint. The rack shaft is accommodated in a rack housing. The ball joint couples the rack shaft and a tie rod to each other. The ball joint includes a ball and a socket. The ball is formed at an end of the tie rod. The socket accommodates the ball. A regulating surface, which is an end face for regulating the movement of the rack shaft, is formed at an end of the rack housing. A regulated surface, which is an end face facing the regulating surface of the rack shaft, is formed on the socket.
The rack shaft moves in a direction while the regulating surface of the rack housing and the regulated surface of the socket are not contacting each other. The regulated surface of the socket thereby approaches the regulating surface of the rack housing. The regulated surface of the socket then comes into contact with the regulating surface of the rack housing. The movement of the rack shaft coupled to the socket is thereby regulated.
According to tests conducted by the inventors of the present application, the steering device may generate a loud abnormal noise due to a stick-slip. The following is considered as the reason therefor. The regulating surface of the rack housing and the regulated surface of the socket are brought into contact with each other while a force of deflecting the rack shaft is acting on the rack shaft. When the movement of the rack shaft is regulated, the regulated surface of the socket is strongly pushed against the regulating surface of the rack housing. Such force is, for example, generated based on a reactive force the rack shaft receives from a turning wheel side through the tie rod when the turning wheel is turning.
When the regulated surface is formed through processing on the socket at a manufacturing stage of the ball joint, a pointed portion pointed toward the regulating surface side of the housing may be formed on the regulated surface due to processing errors.
A socket including such pointed portion is thus formed. In the steering device in which the ball joint including such socket is connected to the rack shaft, the pointed portion of the socket bites into the regulating surface of the housing when the regulated surface of the socket is strongly pushed against the regulating surface of the rack housing as described above.
The rack shaft deforms thereafter so as to eliminate the deflection of the rack shaft. Accompanying therewith, the pointed portion of the socket is moved on the regulating surface of the rack housing while contacting the regulating surface. The stick-slip thus occurs, which produces abnormal noise. The problem has been discussed using the steering device as an example. However, the problem similar to the above is assumed to arise in a steering system including a shaft that moves in a longitudinal direction and a ball joint connected to the shaft.